Would tanks or small walkers be better for Lunar militaries?

Question

This question is similar to my other question "would walkers work on airless worlds?", however even though it's the same setting and I am talking about the same battle group tactic that question dealt with the big boys of war, the vehicles so big it would scare a "Deserts Of Karakh" tank Commander (I don't know the exact size of the things), this question is about vehicles on the size order of an m1 Abrams or AT-RT (two legged walker about half the size of an AT-ST), the main purpose of these is as anti-infantry or ant-anti-Walker vehicles. So a far different discussion.

So for a bit of background on the scenario: the earth-moon system is engulfed in the greatest war it has ever seen, a war waged between powers on both worlds over the fate of the earth-moon system. That's all I have so far... I am talking about fighting on the moon in this case but it applies to any space-rock without an atmosphere.

Since I am trying to figure out the small boys I am simply trying to figure out which is better, tanks or small walkers. They're mostly anti-infantry or anti-vehicle so they require the ability to carry either weapon and to be fairly quick. I am currently for small walkers so let me list of the reasons why.

Now the tanks have the advantage that when tread is blown it doesn't normally crash, two-legged walkers do, four-legged ones similar to crab droids would become stationary. But Walkers have one huge advantage, the lunar dust! if you don't know the lunar dust is so fine that it will get into electronics if it gets stirred, the problem is so bad that any wheeled vehicle would likely stir up so much dust that it could explode because of the amount of dust in it, consider multiplying the amount of deadly dust stirred up by a tank and multiply that by the amount of dust hundreds of tanks would stir and you have the big problem of more than half of them blowing up on the way to battle, consider now how reduced the problem is when you walk on legs, and fairly high. A transport walker (like the AT-OT) could fix the problem of the en-route problem but the dust could still destroy many tanks during the battle.

Edit: Credit to TCAT117 for convincing me that in the case of small vehicles tanks are better, even on the moon. my main concern was that the lunar dust would be a big problem, but it wasn't, and that low g would force the tanks to go ridiculously slow, but war is slow. The jury's still out on number two, I'm keeping this on still because if a pro-walker person can prove to me that walkers can easily go faster than tanks the small walkers will be more important than I am currently going to make them. I'm still going to have big ones though.

Edit: I have come up with a bit more on the scenario that should give you a better idea about why these ground forces exist. First of the earth and moon are both separated between many different governments: the earth is split between a few dozen to over 300 separate nations, the moon probably has over 2,000 nations (the exact number isn't important). but the Lunar armies are for this nation on nation combat. If your attacking from earth the combat will be different.

Edit:

So far so good, I am getting alot of answers talking about the uselessness of land vehicles on the moon and if you think you have something to add then please address it, all of the discussion around that however was before the edit above so I would ask that anyone with that viewpoint fit their answers to the more detailed addition to the scenario on edit two.

But if you want to discuss the how different land vehicles compare then it might be helpful to know that the main factor I am considering is the Lunar regoliths ability to destroy vehicles in transit.

Answer 1

Aerospace engineer here, I see there's already an accepted answer but want to chip in some additional points that I don't see made in any of the other answers.

Any vehicle operating on the lunar surface has to deal with several things not normally encountered on Earth:

• Gravity is one-sixth (1/6) that of Earth
• There is no atmosphere
• The temperature in the sun can range up to +300F, while in shadow can drop below -300F
• Very irregular, rocky terrain with wide craters and a thick layer of regolith
• Lunar regolith is extremely fine and abrasive, meaning it gets into everything and damages everything it gets into

Lunar gravity being so low is actually a benefit: vehicles which would be very heavy and lumbering on Earth would be much lighter (and therefore more agile) on the Moon. Variable suspension (think the trick hydraulics in show cars) or jump jets could allow a vehicle to leap across obstacles, which would normally be impossible on Earth.

The lack of atmosphere is a significant problem for any vehicle, because it means that any oils or greases based on volatile petroleum products would evaporate within minutes, and as such would not be suitable lubricants. NASA uses a variety of dry lubricants, but these can be high cost, high maintenance, or simply not as effective as traditional oils.

The massive temperature variation is another good reason to ditch traditional lubricants in favor of dry ones. A follow-on consequence of this temperature swing is that joints or bearings requiring high precision would not function well, due to differences in thermal expansion.

Finally, perhaps the greatest challenge is the combination of terrain and lunar soil. Wheeled vehicles are mainly designed for smooth surfaces and can only handle rough terrain with advanced suspension systems like those used in the Mars rovers. Traditional rubber wheels also don't work on the Moon due to the afore-mentioned atmosphere and temperature issues, so you need to use something else like metal mesh or plastic, which limits their top speed and load-bearing capacity. Tires also can't be too skinny, or they'll sink into the thick regolith and get stuck.

Tracked vehicles are better able to handle irregular terrain and they spread their weight across a much larger footprint, so they aren't likely to sink. But the drawback with tracks is that every single tread is double-jointed (front and rear) and track systems usually require half a dozen wheels on each side, or more. That's a lot of bearings to lubricate, and a lot of places for regolith to get inside and destroy the joints. And unfortunately, there's no way to keep the regolith out of those joints since they'll all be in constant, direct contact with the ground.

Assuming you've solved the problem of controlling multiple legs (or have a decent supply of Handwavium), legged vehicles have substantial advantages over both wheeled and tracked vehicles in this environment. They have far fewer joints than a tracked vehicle (3-5 per leg) and those joints typically do not need to rotate more than 180 degrees, which means they can be covered with boots to prevent regolith contamination. The feet can be sized for different types of terrain or modes of walking: wide and flat to support a lot of weight without sinking, small and narrow for lighter vehicles or difficult terrain. A legged vehicle can also climb or jump over obstacles that would be difficult for tracked or wheeled vehicles to traverse (here, 6 or more legs is beneficial). Legged vehicles may also have the ability to right themselves if tipped over (assuming the legs have sufficient range of motion), which most modern wheeled or tracked vehicles cannot do.

The biggest benefits I see with using tank tracks are increased speed over smooth terrain, reduced chance of bogging down in thick regolith, and lower vehicle profile. The biggest drawbacks I see are extremely high maintenance requirements (due to wear on the treads and wheels), and mediocre mobility in very rocky terrain.

The biggest benefits I see with using legs are increased speed over rocky terrain, low maintenance requirements (very few of the joints are in direct contact with the lunar regolith), and self-righting capability. There is also the additional bonus of redundancy if you have a 6-or-more-legged vehicle; if one leg is destroyed, the others can reconfigure to permit continued (albeit impaired) movement. The biggest drawbacks I see are high system complexity (independent motors for joint articulation and a sophisticated computer to correctly operate the legs), a need for some precision joints, and higher vehicle profile.

Given the comparison above, personally, I'd go with a walker. Besides the performance and maintenance benefits, it would be pretty awesome to skitter across the lunar surface in an armored cockroach carrying a big gun. ;)

Additional resources on the properties of lunar regolith: https://curator.jsc.nasa.gov/lunar/letss/regolith.pdf

https://www.universetoday.com/20360/lunar-regolith/

http://adsabs.harvard.edu/full/1973LPSC....4.1159T

Answer 2

I was an Anti-tank gunner for 8 years with the Marines, I'll let ya in on why walkers will never be used by a military as an armored vehicle.

First, joints are huge weak points. Not just to enemy fire, but also from a maintenance perspective, and the sensitive servos and gyros required will be infinitely more vulnerable to dirt than anything on a tank. Next, a walker is by nature of being able to walk going to expose more surface area to enemy fire. A good armored vehicle is low to the ground for a reason. Armored vehicles make heavy use of terrain as cover in an attempt to present as small of a target profile as possible. Modern anti tank guided missile weapons basically mean literally anything from infantry to a small drone aircraft can be packing enough heat to destroy a main battle tank. This is why armored vehicles are going to be trending towards smaller and faster instead of bigger and heavier. Finally there is gravity, the moon's gravity makes any sort of walking extremely difficult, this only becomes more pronounced the larger the vehicle is. A large walker could be knocked over too easily in low G to be useful.

Dust is not something that is going to make a vehicle explode. We've been fighting a war in the harshest deserts on our planet for 16 years and our modern tech can deal with sand and dust just fine. Dust will jam up the joints and servos on a walker though.

Answer 3

From first principles: if you have got big squared away, then one could argue small should be as small as possible - 1 soldier. There is a continuum in scifi between powered armor suits and small mechs / walkers. For example, this thing from District 9. Big powered armor or a small walker / mech?

https://www.youtube.com/watch?v=m7dizLdsZJw

The nice thing about small 1-person powered armor / mechs is that they could move by jumping, take advantage of the low lunar gravity. I do not recall much jumping in the Starship Troopers movie but in the book they wore powered armor that let them travel fast by jumping great distances. That would be great for the lunar terrain.

An infantry unit advancing with a series of bounding jumps would be pretty cool. They could ride on the big walker and then explode off of it like fleas.

Answer 4

I would suggest something comparable to the LAV-25 used by the canadian and US military would be more appropriate than either.

Do not underestimate Regolith, Regolith is not mere dust or sand.

On Earth we have erosion effects from wind, rain, weather in general and if nothing else, simply motion. on the moon, we do not. The regolith is entirely composed of tiny abrasive chunks of silicate, razor sharp in almost every tiny grain. That stuff is horrible, it clumps and sticks together, it's statically charged and will stick to just about anything, and if you breath it you might as well be breathing knives, it will shred your lungs and kill you.

Lots of moving parts exposed to vacuum and regolith is going to be an awful maintenance headache on its own. Wheels and motors on a relatively lightweight mobile platform will reduce that enormously. the many independent drive systems mean that if anything jams up it'll be able to keep going, and the 8 wheels will spread the weight more than sufficiently in the lower gravity.

Legs have been discussed to death, the only practical reason I can see for using them on the moon is to avoid dust by literally stilt-walking through it. The terrain on the moon is not rough enough to merit legs even if the gravity makes them easier to construct and move around with.

Treads meanwhile are 100% exposed to regolith and essentially guaranteed to clog up with it. The big draw of treads is the hugely effective weight-distribution and surface-grip. but in lower gravity you don't need the weight distribution nearly as much, I couldn't comment on the grip issue.

There is very little that can't be done by a wheeled platform instead of a conventional tank when mass is a reduced issue.

For an example in science-fiction, you might look at the Mako vehicle from Mass Effect. A futuristic version of the LAV-25 I mentioned at the start.

Lunar combat is going to be a messy affair, a nightmare combination of knife-fight range direct combat and satellite guided munitions.

Your horizon is nearer than on earth, likely nearer than it should be simply because of the craters, you have no over-the-horizon radar (on earth, we achieve this by bouncing radar off the ionosphere) your only way to track things beyond the horizon or even line-of-sight is by satellites and flying craft that can directly see them.

You may be able to achieve limited long range sensors by observing surface-vibrations from ground vehicles moving around, but in all likelihood the regolith will act as a vibration absorber, another argument for avoiding legs as the pounding footsteps will alert any such sensors like the vibrating glass of water in Jurassic Park.

Explosions, particularly fragmentation weaponry will be murderous, on earth a frag grenade is intended to injure as much as it is to kill, on the moon, any suit-puncture is likely to be immediate death without some seriously clever automatic sealant technology, even then the task of getting back to a pressurised environment is going to take long enough moon-warrior is likely to die of their injuries.

Any infantry operating on the surface will definitely want some form of armoured vehicle support, if nothing else to provide a med-evac and triage.

Answer 5

It depends entirely on how agile your walkers are - if they're basically tanks on legs because of rule of cool, then ordinary tanks are going to be much more sensible and feasible due to reasons elucidated in other answers. In particular, survivability is a big plus for tanks - apart from incredibly thick armor, hard-kill active protection systems (APS) are becoming a de facto standard on newer models, which allows the tank to automatically detect and destroy incoming threats that would have been fatal to previous generations of vehicles. Essentially, tanks are becoming mobile fortresses that are mostly impervious to anything except aircraft and enemy tanks of equal capability.

On the other hand, if your walkers are capable of jumping, dodging, and frustrating enemies by being generally too nimble to hit with weapons strong enough to bring them down, then you have a case. The low gravity, coupled with reaction jets, would make them even more maneuverable. Armor would be minimal (protection against small-arms fire at best) because (a) one hit from a high-calibre weapon and the unit is out of the fight anyway (b) more armor would affect agility, which is this unit's most important attribute.

The walker's joints being a weak spot is a moot point because the assumption is that the enemy forces are having a hard enough time hitting the main body of the walker, let alone its limbs.

For weapons, it depends very much on how esoteric you want to get. With a humaniform walker, the conventional approach would likely involve installing a high-calibre weapon akin to a tank's turreted main gun, in place of one of the "arms".

This all assumes, of course, that the enemy up against these walkers doesn't have/isn't using the simplest way to defeat hard-to-hit glass cannons, namely radiation-seeking missiles. (Take for example today's BGM-71E TOW, which can [theoretically] be carried and launched by infantry and is capable of defeating up to 900mm of reactive armor.) Presumably the faction using walkers would be acutely aware of this vulnerability and utilise tactics to minimise it, e.g. by trying to get their enemy to fruitlessly expend the ammunition of this weapon.